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We use Barrett tables (misspelled in the code unfortunately) to fold crc computations of multiple buffers into a single crc. This is important because it turns out to be faster to compute crc of three different buffers in parallel rather than compute the crc of one large buffer, since the crc instruction has latency 3. Currently, we have a separate code generation step to compute the fold tables. The step generates a new C++ source files with the tables. But modern C++ allows us to do this computation at compile time, avoiding the code generation step. This simplifies the build. This series does that. There is some complication in that the code uses compiler intrinsics for the computation, and these are not constexpr friendly. So we first introduce constexpr-friendly alternatives and use them. To prove the transformation is correct, I compared the generated code from before the series and from just before the last step (where we use constexpr evaluation but still retain the generated file) and saw no difference in the values. Note that constexpr is not strictly needed - we could have run the code in the global variables' initializer. But that would cause a crash if we run on a pre-clmul machine, and is not as fun. Closes #11957 * github.com:scylladb/scylladb: test: crc: add unit tests for constexpr clmul and barrett fold utils: crc combine table: generate at compile time utils: barrett: inline functions in header utils: crc combine table: generate tables at compile time utils: crc combine table: extract table generation into a constexpr function utils: crc combine table: extract "pow table" code into constexpr function utils: crc combine table: store tables std::arrray rather than C array utils: barrett: make the barrett reduction constexpr friendly utils: clmul: add 64-bit constexpr clmul utils: barrett: extract barrett reduction constants utils: barrett: reorder functions utils: make clmul() constexpr